Collagen is a fibrous protein that occurs in vertabrates as a chief constituent of connective tissue. A number of different collagens have been derived from various sources, such as skin, bone, cartilage, etc. Collagens, in general, are insoluble in aqueous media; however, collagens are known to exist which are soluble. Other collagens may be solubilized by various methods including pH adjustment, heat treatment, reduction, etc. For example, Collagen I, the most common collagen found in the skin, occurs naturally in fibers made up of cross-linked triple-stranded helices. Removal of the cross-linked regions by pepsin treatment and/or unwinding of the strands by heating leads to greater solubility. Means of distinguishing solubilized fragments of insoluble collagens is of utility in the study of collagens.
Gelatin, a product obtained from prolonged heating of collagen-containing tissue, is known to bind with fibronectin, an extracellular matrix and blood glycoprotein, and it is known that fibronectin can be readily isolated by chromatography on an affinity matrix prepared by coupling gelatin to a solid phase (Engvall and Rusolahti, Int. J. Cancer 20 1977), 1-5). It would seem that soluble or solubilized collagens could be isolated by a reverse affinity chromatography procedure in which fibronectin is coupled to a solid support. Fibronectin, however, in addition to having sites which selectively bind with collagen, has sites with affinity for a number of other high molecular weight proteinaceous substances including glycosaminoglycans, fibrinogen, actin, and surface material of Staphylococci and certain other cells. Furthermore, it has been found that, when intact fibronectin is coupled to certain solid supports, the number of available collagen binding sites is apparently reduced.